The present invention is related to improved ceramic filters particularly suitable for filtration of aluminum and magnesium based alloys. More specifically, the present invention is related to ceramic foam filters with either BN or Y2O3 coatings to improve resistance to chemical reactivity.
Commercial aluminum-lithium (Al—Li) alloys are used primarily in aerospace applications because of their low density, high specific modulus, excellent cryogenic toughness and superior fatigue crack growth resistance. Each 1 weight % addition of lithium can reduce the density of an AL-Li alloy by about 3% and increase its stiffness by about 5%.
Commercial magnesium alloys are finding acceptance in the transportation industry and particularly the automotive sector. Magnesium alloys typically have a lower density than aluminum alloys and have high specific stiffness.
It is typically desirable to filter the alloy during the molten state to remove either solid or liquid insoluble second phase impurities. Ceramic filters are used widely in removing inclusions from molten aluminum alloys. Traditional ceramic foam materials used for filtering aluminum alloys include aluminum phosphate bonded alumina, sinter-bonded alumina and boron glass bonded kyanite. Lithium is one of the most reactive alkali metals and will reduce almost any of the common oxide materials, except yttrium oxide and calcium oxide, at high temperature according to the Ellingham free energy diagram. Corrosive attack by lithium alloying additions is accelerated by the high vapor pressure of lithium. Corrosion attack of the filter material can occur before the onset of wetting due to lithium vapor infiltration into the filter microstructure. When added to molten aluminum, lithium has a high tendency to react with oxygen and nitrogen at the molten metal-air interface and to form lithium oxide, lithium aluminate and lithium nitride inclusions. Lithium oxide destabilizes the normally protective aluminum oxide film on the molten aluminum surface, resulting in accelerated oxidation and the formation of inclusions. Traditional ceramic foam materials, including sinter-bonded alumina, when used in Al—Li alloy filtration, are severely attacked by reactive lithium, and can break down in the filtering process, potentially resulting in filter fragments and inclusion releases from the filter structure. The chemically attacked ceramic filter surface eventually becomes wetted by the molten Al—Li alloy due to surface reactions and filter surface starts to corrode away. When this occurs, inclusion adherence and stable separation of inclusion material at the surface will no longer occur.
Calcium oxide should not be reduced by lithium based on the Ellingham free energy diagram and should be stable in aluminum lithium alloy. Unfortunately, calcium oxide is very hydroscopic and a filter made of calcium oxide will likely react with moisture in air to form calcium hydroxide, making the filter un-useable. Magnesium oxide according to the Ellingham Free Energy diagram should be stable in Al—Li alloys. However, if the starting MgO powders used in the filter slurry contain a significant level of impurities, the resulting MgO filters may have reduced corrosion resistance to molten Al—Li alloys.
Magnesium alloys are also very reactive in the liquid state. Magnesium also has a high vapor pressure and the reactivity of the vapor is enhanced relative to the liquid state. Sinter-bonded alumina, aluminum phosphate bonded alumina, boron glass bonded kyanite and zirconium oxide ceramic filters cannot be used in molten magnesium alloy filtration due to the aggressive reactivity, particularly, of the magnesium vapor.
There is a need for an improved material for filtering aluminum and magnesium based alloys and particularly Al—Li alloys, Mg-containing aluminum alloys and other reactive alloys such as titanium and titanium aluminides.